1. Field of the Invention
The field of the invention relates to a demodulating and digital decoding method and apparatus for demodulating and decoding a quadrature amplitude modulated or a vestigial sideband modulated digital data signal permitting utilization of substantially the same apparatus for demodulating either form of modulation.
2. Description of Related Art
FIG. 1A is repeated here from applicant's copending U.S. application Ser. No. 08/223,223, filed Apr. 5, 1994, entitled Synchronous Detector and Methods for Synchronous Detection, incorporated herein by reference, to show a typical signal amplitude versus frequency density spectrum of an information signal modulated via a vestigial sideband modulation scheme. In particular, FIG. 1A shows a vestigial sideband (VSB) modulated information signal modulated about a carrier frequency f.sub.c. In particular, the carrier frequency signal is practically absent, the frequency component at the carrier frequency being primarily the information signal. The slopes on either side of the information signal frequency spectrum versus amplitude are ideally identical.
Referring to FIG. 1B, a known quadrature amplitude modulated (QAM) signal has a frequency spectrum which differs from that of FIG. 1A in that the QAM carrier frequency signal is symmetrically placed midband between f.sub.c shown in FIG. 1A and f.sub.c +(f.sub.b /2) where f.sub.b is the baud rate of the modulated digital data information signal. The QAM modulated information signal is demodulated assuming a center carrier frequency f.sub.cs, where "f.sub.cs " refers to the frequency value equal to the approximate center of symmetry of the information signal spectrum.
In a conventional demodulator for a VSB modulated data signal in which the transmitted carrier is suppressed or partially suppressed, the demodulation apparatus typically includes a synchronous product detector tuned to the carrier frequency f.sub.c. In a typical demodulator for the QAM data signal, the demodulation apparatus involves the use of two product detectors, one tuned to f.sub.cs, or f.sub.c +(f.sub.b /4), and the other to the quadrature of f.sub.cs.
As television technology has progressed toward today's required resolution requirements permitting display of a high definition television image, it has become necessary to develop and agree upon a modulation scheme for a digital data signal capable of transmission via cable, optical fiber, satellite or over-the-air at very high, ultra-high, microwave or higher radio frequencies (including light frequencies). The transmitted digital data signal, itself, includes both a digitized and compressed television information signal and a related stereo audio signal and further may include other data signals. These other data signals may be superimposed on either of the television or stereo audio signals or separately transmitted in a frequency division or time division multiplex format. The modulation techniques presently under consideration are the subject of considerable controversy and include both VSB and QAM. Some proponents of one high definition television standard are promoting VSB while others are promoting QAM. For example, 16-QAM is a technique where a sequence of four bit data nibbles are split into two separated sequences of two bit symbols per sequence. The two separated sequences of symbols are fed into the two modulation ports of a quadrature type modulator. QAM output signals are double sideband (as per FIG. 1B), and the sidebands bear no particular phase relationship to each other due to asymmetry between the two separate sequences of symbols formed during the modulation process.
In contrast, 4-VSB is a technique where the same sequence of four bit nibbles is split in two consecutive two bit nibbles transmitted at twice the symbol rate used in QAM. The bandwidth is made comparable to the QAM modulated signal's bandwidth by eliminating one redundant sideband.
The known modulation and demodulation methods vary between QAM and VSB. So does the structure of a typical modulator circuit or a demodulator circuit. One synchronous detector for VSB demodulation is shown in FIG. 2 substantially copied from FIG. 24 of applicant's copending U.S. application Ser. No. 08/223,223. The VSB demodulator typically varies from one for QAM demodulation in the requirement to tune to different frequencies f.sub.c versus f.sub.c +(f.sub.b /4)) and to maintain control of the one or the other tuned frequency output of the local carrier voltage-controlled oscillator (VCO).
There is reason to believe that representatives of CableLabs, a cable television research and testing facility in the United States, have sought to develop a chip set that merges QAM and VSB modulation technology. The CableLabs modulation and demodulation method and chip set under development may have resulted in the preparation and filing of patent applications in the United States according to press releases. However, the particular details of the CableLabs development have not been published to the best of applicant's knowledge.
Consequently, in view of the controversy surrounding digital VSB and QAM demodulation, there remains a need in the art to provide apparatus and a method for digitally demodulating both types of modulation and thereby decoding a digital information signal modulated thereby. Desirably, such apparatus will avoid any need to retune to an anticipated carrier frequency, eliminate redundant circuitry and, yet, decode either modulated digital data signal without significant loss of content.